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Heat oven to 190C/fan 170C/gas 5. Butter and line the base and sides of a 30 x 35cm Swiss roll tin with baking parchment.
Separate the eggs, putting them into two large mixing bowls. Add the sugar and 2 tbsp water to the egg yolks. Using an electric whisk or tabletop mixer, whisk the sugar and yolks for about 5 mins until the mixture is light in colour and thick enough to leave a trail when the whisk blades are lifted. Sift in the flour and cocoa, then fold in lightly, using the whisk blades or a large metal spoon.
Using a clean whisk, beat the egg whites until they are stiff, then fold into the cake mixture in three batches, again using the blades to preserve as much air as possible.
Pour the mixture evenly over the prepared tin, then carefully spread to the edges. Bake for 10-12 mins, or until the cake feels firm to the touch. Put a large sheet of baking parchment on the work surface and sprinkle lightly with caster sugar. Turn the cake out onto the parchment and peel off the lining paper. Cover with a clean tea towel, then leave to cool completely. Trim a little cake from all the edges, then score along the edge of one of the long sides of the cake and roll up from there, using the paper to help you, rolling the paper inside the cake.
To make the icing and filling, bring the cream to the boil in a small pan, remove from the heat, then break in 400g of the chocolate. Stir until it is melted and smooth. Leave to cool, then chill until it is spreadable, about 1 hr. Chop the remaining chocolate, spoon a third of the icing into a bowl, then stir them together to make the filling.
Carefully unroll the cake, then spread all the filling over to within 2cm of the edges. Roll up the cake again using the paper to help you, then set on a board.
Cut a thick diagonal slice off one end of the cake. Transfer the larger piece of cake to a board or flat serving plate. Spread a little icing over cut side of the small piece of cake and fix it to the large roll to make a stump (as photo). Spread remaining icing over the cake, then dust with icing sugar to serve.